1. Field of the Invention.
The present invention relates to a calender press for use in a paper-making machine, and, more particularly, to a calender press providing locally adjustable nip loading between a top roll and a bottom roll.
2. Description of the Related Art
A paper-making machine includes a wet end, a forming section, a press section, a dryer section, a calender stack, a reel and winder. The calender stack typically is in the form of a plurality of rolls which are stacked in substantially vertical alignment relative to each other. The paper web passes through at least one nip formed by adjacent rolls in the calender stack and is provided with a desired surface finish by the calendering operation.
For a wide paper machine (i.e., a paper machine forming a paper web with a width between approximately 200 to 480 inches), the rolls of a calender stack are typically formed of metal and have a wall thickness of between approximately 3 to 4 inches. The thick walls inhibit sagging of the long roll. It is known to locally adjust the nip load between adjacent rolls by providing pistons within one of the rolls which exert a radially outward force on the inside diameter of the roll, thereby locally deflecting the roll to provide local adjustment of the nip load. Such a piston arrangement works well to locally adjust the nip load on paper grades where a high nip load is required or acceptable (e.g., at least 700 lbs. per linear inch (pli)). However, at lesser nip loads it has been found that the force exerted by the pistons is insufficient to locally deflect the thick wall of a long calender roll. Thus, it is not possible with conventional designs to provide for local adjustment of the nip load on calender rolls having thick walls at low nip loads.
With light weight papers, such as tissue, etc., nip loads of greater than approximately 700 pli in a calender stack may cause xe2x80x9cblackeningxe2x80x9d or dark spots to be formed on the paper web. Since light weight paper grades cannot be calendered at greater than 700 pli, and since local adjustment of the nip load is not possible with conventional calender stacks at less than approximately 700 pli, it is apparent that conventional calender stacks do not allow for local adjustment of the nip load when calendering light weight paper grades.
It is also known to apply heat to rolls in a calender press to adjust for temperature fluctuations in the roll resulting from local or generalized cooling of the roll because of contact with the paper web.
What is needed in the art is a calender which allows local adjustment of the nip load in a calender press on machines forming light weight paper with a wide paper web.
The present invention provides a calender press with a top roll and a bottom roll which are each crowned and thermally compensated in a zone-wise manner across the length thereof to provide locally adjustable nip loading at relatively low nip loads.
The invention comprises, in one form thereof, a calender press for use in a paper-making machine. The calender press includes a top roll positioned adjacent to a nip and rotatable about a longitudinal axis thereof. The top roll has a length and circumferential perimeter. The length extends in the direction of and is longer than the width of the fiber web. The top roll is thermally compensated in a plurality of thermal zones which are adjacent to each other across the length of the top roll, whereby the perimeter of the top roll may be adjusted in a locally adjustable manner toward and away from the nip. A bottom roll is positioned adjacent to the top roll and defines the nip with the top roll. The bottom roll is rotatable about a longitudinal axis thereof and has a length and circumferential perimeter. The length extends in the direction of and is longer than the width of the fiber web. The perimeter is smaller at each longitudinal end of the bottom roll and larger at approximately a midpoint between the longitudinal ends, thereby defining a crowned bottom roll. The bottom roll is thermally compensated in a plurality of thermal zones which are adjacent to each other across the length of the bottom roll, whereby the perimeter of the bottom roll may be adjusted in a locally adjustable manner toward and away from the nip. The thermal compensation of the bottom roll and the top roll coact to effect a nip load of less than approximately 150 pounds per linear inch between the top roll and the bottom roll.
An advantage of the present invention is that locally adjustable nip loads can be provided between the top roll and the bottom roll at relatively low nip loads.
Another advantage is that the locally adjustable nip loads are provided through thermal rather than mechanical deformations of the top and bottom rolls.